Technical Field
The present disclosure relates to a high-electron-mobility transistor (HEMT) of the normally off type including a trench, which comprises a gate region and forms at least one step; further, the present disclosure regards the corresponding manufacturing method.
Description of the Related Art
As is known, HEMT transistors, which are also known as “heterostructure field-effect transistors” (HFETs), are encountering wide diffusion, since they are characterized by the possibility of operating at high frequencies, as well as on account of their high breakdown voltages.
For instance, HEMT transistors are known that include AlGaN/GaN heterostructures, which, however, are devices of a normally on type, i.e., such that, in the absence of voltage on the respective gate terminals, there in any case occurs passage of current; equivalently, these transistors are said to operate in depletion mode. Since it is generally preferable to provide transistors of the normally off type (equivalently, operating in enrichment mode), numerous variants have been proposed, such as for example the transistor described in U.S. Pat. No. 8,587,031.
In detail, U.S. Pat. No. 8,587,031 describes a transistor including a heterostructure of a layer of aluminum gallium nitride (AlGaN) and by a layer of gallium nitride (GaN), arranged in contact with one another. Further, the transistor has a first gate region, which is arranged within a recess that extends in the AlGaN layer and enables modulation of a channel of the normally off type.
Today, there are thus available HEMT transistors operating in enrichment mode. However, these solutions are in any case affected by the so-called phenomenon of drain-induced barrier lowering (DIBL), also known as “early-breakdown phenomenon”.
Unlike breakdown, the DIBL phenomenon occurs for low drain-to-source voltages (typically, for voltages comprised between 10 V and 30 V) and entails, in the presence of a zero voltage between gate and source, a sudden increase of the current that circulates between the drain and the source. In greater detail, denoting the voltages present between i) the gate and the source and between ii) the drain and the source as the voltages Vgs and Vds, respectively, and the current that circulates between the drain and the source when Vgs=0 as the leakage current, when Vds<Vdib1 (where Vdib1 is the voltage at which the DIBL phenomenon occurs) the leakage current density is typically of the order of nanoamps per millimeter. Instead, if Vgs=0 and Vds exceeds Vdib1, the leakage current density may even be of the order of the microamps per millimeter. Since the DIBL phenomenon causes premature turning-on of the transistor, there is felt the need to prevent onset of this phenomenon, or in any case reduce the effects thereof.